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Description
Flotation is used to separate particle intermixtures in many industrial applications, most notably in the mining industry for extracting valuable minerals from mined ores. During the flotation process, particles attach to air bubbles based on their hydrophobicity. More efficient recovery of fine and ultra-fine particles (smaller than 10 µm) benefits from highly turbulent flows and smaller bubble sizes.
We present results from a two-phase model experiment of a pressurised pneumatic flotation cell. The cell is constructed based on the principles of the Concorde Cell™ (Metso, see Jameson, Miner. Eng. 23, 835–841, 2010 and Yáñez et al., Miner. Eng. 206, 108538, 2024). In a vertical pipe, air bubbles are generated by a plunging jet: a water jet impinges on a free water surface, entrains the surrounding air and generates sub-millimetre sized bubbles. These are transported downwards to the lower tip of the pipe, where a nozzle accelerates the flow and ejects a bubbly jet into a water-filled cell. Due to the pressure drop over the tip nozzle, the air surrounding the plunging jet has to be pressurised.
The properties of the generated bubbles and their evolution along the downcomer height are investigated by recording images of the bubbly flow using optical shadowgraphy. The bubble size is determined with a machine learning algorithm to segment the bubbles in the shadowgraphs. Control parameters of the process are the water and air flow rates, as well as the surfactant concentration in the water. The generated bubbles are found to considerably decrease in size for higher surfactant concentrations up to a certain critical concentration, beyond which the size stays constant. Along the height of the downcomer, it can also be verified that the surfactant successfully suppresses coalescence. The bubble size increases slightly with the air flow rate and decreases with the water flow rate. Both dependencies can be collapsed by expressing the bubble size as a Weber number.
